Scientists Record World's Largest Raindrop

December 7, 1986|By James Gleick , New York Times

NEW YORK — Flying above the volcanic coast of Hawaii, buffeted by a tropical storm, a team of cloud physicists armed with laser and computer has recorded what may be the largest raindrop ever measured.

It was 8 millimeters across, almost the size of a dime. Oscillating in the wind, it left its shadow on an array of photodiodes carried by an airplane belonging to the National Center for Atmospheric Research.

Along with a brigade of other drops in the range of 4 to 6 millimeters, it has convinced scientists that rainstorms create drops considerably larger than theorists had thought.

''There was a kind of consensus that raindrops would never get bigger than about two and a half millimeters,'' said Kenneth V. Beard of the University of Illinois at Urbana-Champaign. ''We found amazingly large drops, and we decided that we had been misled by calculations based on laboratory studies and on earlier experiments. They the earlier experiments were just sitting on the ground and taking whatever nature blew by them,'' Beard said.

The discovery has forced scientists to rethink some fundamental ideas of how raindrops are formed in the hot, moisture-laden air of a tropical cloud and how surface tension and air turbulence hold drops together or rip them apart.

Beard and his colleagues, David B. Johnson and Darrel Baumgardner, also have been struggling to explain the survival of large drops in the face of frequent opportunities for collisions. They estimate the average lifetime of a 5-millimeter drop to be 23 minutes.

And beyond its theoretical significance, the average size of raindrops matters to engineers responsible for calibrating weather radar. Such radar is often used to estimate the amount of rain falling at places off in the distance.

The discovery was part of the Joint Hawaiian Warm Rain Project, a continuation of a tradition of rain research at a site, southeast of Hilo on Hawaii Island, that could be described as a forecaster's paradise, because it rains every day there.

The project benefited from the latest in airborne instrumentation. A laser cast raindrop shadows on electronic photodiodes, and a computer helped guide the airplane back and forth through the regions of heaviest rain.